Extruders are presently used to produce a wide variety of food products through the process of pressurizing and heating a mixture of dry material and moisture, and then forcing the material through a die to form an extrudate after which the extrudate is cut into a pellet or other form by a number of rotating blades. One type of extruder includes a screw which has a varying blade pitch and also a varying depth, the screw being surrounded by a barrel or series of barrels which confine the material as it is pressurized by the screw. The screw moves the material trough the barrel and into a die, the die having a series of holes through which the material is pushed, and a number of blades which are motor driven to rotate about the die cut the material off after it is pushed through the holes. In a typical process, the pressure inside the extruder may be 350 psi and the temperature can be elevated above 400.degree. F. In an extrusion process, these intense pressures and temperatures are important in creating the finished product by both cooking the product, and also providing for an expanded or puffed effect after the product is released through the die into the lower atmospheric pressure.
In any extruder, it is important to operate the extruder such that the proper temperatures and pressures are attained while also maximizing the throughput of material. In the prior art, the inventor herein is not aware of any automatic extruder controller which attempts to maximize the throughput capability of the extruder. Instead, those controls of which the inventor is aware merely focus on attaining the proper temperatures for the extrudate as it is released through the die. However, there is no appreciation in the prior art for the effect of this temperature as well as pressure, and their relationship to the throughput which may be attained. The inventor believes that as the temperature and pressure rise in the extruder, then steam will be formed from the water in the material as it is cooked. Although a substantial portion of the water in the material may be converted into steam without affecting the extruder process, as the liquid moisture levels reach a minimum value, there is a tendency for the extruder to choke or overload because of the increased friction caused by the reduced lubrication in the material, as well as a solidification of the material itself. Therefore, this factor limits the maximum throughput capability of the extruder. To avoid this choking, various actions can be taken. These include lowering the temperature of the extruded material by either adding cooling water to the last barrel of the extruder or adding greater amounts of water to the mash itself as it is input to the extruder. If the temperature drops too low, then steam can be added to the last barrels of the extruder to elevate the temperature of the mash, or steam may be directly added to the mash as it is input to the extruder.
To take advantage of these operating characteristics of an extruder and to automate the extruder to increase its throughput without deleteriously affecting the finished product or extrudate, the inventor herein has succeeded in developing a control which monitors the exit temperature of the extrudate and the pressure at several places along the length of the extruder and uses these measured parameters to control the input of dry material and moisture for the mash, as well as directly controlling the temperature of the last barrel section by controlling the flow of water or steam through a jacket surrounding it. It is believed that with the inventor's controller, an extruder may be operated at or near its maximum throughput while producing a "cooked" extrudate which has been processed at the proper temperatures and pressures. Furthermore, by measuring the pressure within the extruder, the thermodynamic point at which water changes into steam may be carefully calculated and utilized in operating the extruder. As the extruder has a tendency to wear over its operating life, the continuous pressure measurements can take account of that fact to maximize extruder throughput and finished product despite the wear of the extruder.
While the principal advantages and features of the extruder controller have been described above, a fuller understanding may be obtained by referring to the drawings and description of the preferred embodiment which follow.